The president of Brazil's Senate was put on the defensive on Wednesday with the release of a secretly recorded conversation that shows him proposing to weaken one of the key tools prosecutors have used to trap politicians and businessmen in a sweeping corruption scandal.

The released transcript and audio, published by the Folha de S. Paulo newspaper, appears to feed widespread suspicions that top lawmakers are trying to stifle investigations into billions of dollars in alleged bribes involving the state oil company, Petrobras.

The conversation shows Senate leader Renan Calheiros suggesting legal changes to bar the use of plea bargains with people who have been arrested. Prosecutors have used such deals, offering lighter sentences to convince suspects to implicate high-ranking businessmen and politicians, including Calheiros himself.

"You can't make a plea deal while arrested. That's the first thing," Calheiros allegedly said in a wide-ranging conversation about the Petrobras probe with Sergio Machado, an ex-senator and former head of another state oil company, Transpetro, who also is being investigated.

SEVEN ALLEGATIONS

Brazil's Supreme Federal Tribunal, the country's highest court, is currently considering seven investigations against Calheiros in the Petrobras probe.

Among the cases are accu-sations by a lobbyist that Calheiros was paid US$600,000 to stop a Senate probe of corruption in Petrobras. He denies wrongdoing in all cases and he has not been charged.

The newspaper did not say when or by whom the recording was made. However, on Wednesday the Supreme Federal Tribunal announced that Machado had reached a plea bargain with prosecutors, making it a strong possibility that he made the recording.

The court did not provide details of the deal, and Machado's whereabouts were not immediately known.

Calheiros' office issued a statement acknowledging the conversation, but insisting that the talks "don't reveal, indicate or suggest any mention or attempt to interfere with" the Petrobras probe.

ANOTHER LEAK

The newly leaked transcript came two days after another leak, also involving Machado, raised suggestions that this month's impeachment and suspension of President Dilma Rousseff was part of an attempt to stifle the Petrobras investigations.

That audio revealed Romero Juca, a senator who was part of the Cabinet named to replace Rousseff's, strategising with Machado about impeaching Rousseff to stop "the bleeding". Juca said he was taking a leave of absence within hours after the conversation was released.

Rousseff has repeatedly said the allegations against her, that she broke fiscal rules in managing the federal budget, are cover for a "coup" aimed partly at limiting the Petrobras probe.